Between 22 and 24 million cattle are slaughtered annually in the United States. Of these, over half are processed by high-output meat packing plants using state-of-the-art technology. Despite modern processing procedures, bacterial contamination of fresh meat occurs as an undesirable but unavoidable result of converting live animals into food. Although current Good Manufacturing Procedures (GMPs) can reduce this contamination, the total elimination of bacteria from fresh raw meats has not been achieved. Since deep muscle tissue is considered sterile in healthy animals, essentially all of the contamination originates from the carcass surfaces. Soil, dust and manure on the hides and hooves of the animals represent the primary source of contamination. In addition, in rare cases internal organs rupture during the removal process, resulting in further contamination of the carcass.
Conventional slaughtering operations are designed for high volume processing. The animals are herded from holding pens through inclined walkways into the plant. Once in the plant, the animal enters a restricted area with a pivoting floor. The animal is then stunned, typically with a captive bolt stunner, and the floor of the restricted area pivots, sliding the animal onto the processing floor. One of the rear legs of the animal is shackled, and the shackle is hooked to the processing rail. The animal travels (head down) along the rail to the bleeding, or "sticking" area, where the carotid arteries and jugular vein are severed. Since the heart is still beating, the blood in the animal drains out very quickly, and the animal is technically dead within one minute. The entire time interval from stunning to death is typically less than five minutes.
The carcass advances from the bleeding area into the processing area. During the initial stages of processing, the hide is peeled from the shackled leg and the front feet are removed. As the carcass continues along the rail, a circular incision is made around the anus, which subsequently drops into the body cavity. Part of the hide is trimmed back from the free hind leg and from the rump area, including the tail, and an incision is made from the hindquarters of the carcass down the belly to the head. The incision is expanded along the underside of the front legs, and the hide is partially trimmed back from the front legs. The hind feet, head and external sexual organs are also removed during the initial stages of the process. The carcass continues down the rail to a mechanical hide puller which removes the hide in a single downward motion.
Some meat processing operations also incorporate pre-evisceration washing and/or sanitizing steps prior to evisceration, preferably within an hour after hide removal. (See Clayton et al., U.S. Pat. Nos. 4,852,216 and 4,862,557). Pre-evisceration washers resemble a drive-through car wash; the skinned carcass is rinsed with low pressure (50 psi or less) water using vertically oscillating spray nozzles. The carcass is also optionally treated with a food-grade sanitizer, typically lactic or acetic acid. Pre-evisceration washing removes contaminants from the carcass surface, while sanitizing inhibits or kills any residual bacteria. Despite the reported reduction in bacterial contamination following pre-evisceration washing, many meat processors omit this step because of space limitations, the expense of implementing the washing system and/or the proprietary nature of this technology.
After the optional pre-evisceration washing and sanitizing steps, the carcass is eviscerated, trimmed to remove any visible contamination ("zero-tolerance" trimming) and washed with water under high pressure, typically 400-450 psi. Unlike pre-evisceration washing, final washing is required by the U.S. Department of Agriculture. The washed carcass is typically sanitized with an organic acid (lactic or acetic) rinse. Finally, the processed carcass is chilled, typically using a combination of forced air chilling and spray chilling.
While various cleansing and sanitizing treatments are known to the art, none reduces the microbial contamination of animal carcasses to acceptable levels. All existing treatments attempt to remove contaminants already present on the surface of the carcass. However, many contaminants, particularly metabolically active bacteria, adhere to the surface of the carcass and become recalcitrant to subsequent washing procedures.
A need therefore exists for an efficient and practical method for reducing or eliminating contamination of animal carcasses during slaughtering. More particularly, a need exists for a means for modifying the surface properties of the exposed animal carcass so that fewer contaminants adhere to the surface, or adhere less tightly, thus minimizing contamination and improving the efficiency of subsequent cleansing procedures.